1. Field of the Invention
This invention relates to an improved and simplified diffractive subtractive color filtering technique.
2. Description of the Prior Art
U.S. Pat. No. 3,957,354, issued May 18, 1976 to Knop and assigned to the same assignee as the present application, discloses a diffractive subtractive color filtering technique. More specifically, as taught in U.S. Pat. No. 3,957,354, the illumination of a square-wave diffraction phase grating (or other equivalent binary phase delay diffractive structure) with polychromatic (white) light results in the zero diffraction order light emerging therefrom exhibiting a particular color hue determined solely by the optical amplitude of the phase grating. In principle, such a phase grating may be either a reflective phase grating, or a transmissive phase grating. A transmissive phase grating may be manifested by differences in indices of refraction and/or a surface relief pattern in a transparent medium. However, in practice, the most suitable type of phase grating is a surface relief pattern embossed in a transparent medium, such as a sheet of plastic, having an index of diffraction different from the surrounding air. In the case of such a surface relief pattern in a transparent medium, the optical amplitude of the diffraction grating is equal to the product of the physical amplitude of the grating relief pattern multiplied by the difference between the index of refraction of the transparent medium and that of the surrounding air.
As further taught in the aforesaid U.S. Pat. No. 3,957,354, a diffractive subtractive color filter manifesting color pictures can be achieved by the superposition of three square-wave phase gratings (or other equivalent binary phase delay diffractive structures) which either have different line spacings and/or are angularly-displayed with respect to each other by a suitable angle (e.g. 60.degree.) and each of which has a different preselected optical amplitude corresponding to a different one of the three subtractive primary colors (cyan, magenta and yellow).
As is further taught in the aforesaid U.S. Pat. No. 3,957,354, a conventional slide projector, microfiche viewer or motion picture projector may be employed to display a color picture obtained from a diffractive subtractive color filter. This results from the fact that the effective line frequency of each of the three superimposed diffraction gratings is sufficiently high to deflect all the higher diffraction orders beyond the aperture of the projection lens of the projector, so that only the respective zero diffraction orders of each of the superimposed diffraction gratings passes through the projection lens of the projector and is displayed.
A color picture recorded as three superimposed diffraction gratings (or other equivalent binary phase delay diffractive structures) in the form of one or more surface relief patterns involves eight different depth parameters. In particular, white light is manifested by a zero depth level; cyan light is manifested by the depth of a first of three superimposed gratings; magenta is manifested by the depth of a second of the three superimposed gratings; yellow is manifested by the depth of a third of the three superimposed gratings; blue is manifested by the three depths equal to each of the first and second of the three superimposed gratings and their sum; green is manifested by the three depths equal to each of the first and third of the three superimposed gratings and their sum; red is manifested by the three depths equal to each of the second and third of the three superimposed gratings and their sum; and black is manifested by the seven depths equal to each of the first, second and third of the three superimposed gratings and all possible sums thereof taken both two at a time and all three at a time. It would greatly simplify the fabrication of color pictures in the form of diffractive subtrative filters if it were not necessary to use as many as three independent gratings, superimposed in different combinations to obtain the eight different colors.